First, you need a graphics package. Something like Photoshop, Paintshop or Fireworks is ideal. If you have none of these, you can download the GIMP for free.

Next you need a file converter. This is used to convert Warcraft image files (.BLP) into editable image files (such as .TGA, .JPG, .BMP). Generally, you want to be working with .TGA rather than .BMP or .JPG. Download one or all of Yobgul's file convertor and/or Magos' Model Editor, Warcraft 3 Viewer.

Assumably, you have a copy of Warcraft 3: The Frozen Throne. If you don't, buy it.

You need a program to extract model and texture files from Warcraft archives, known as Mo'PaQ (.MPQ). Download WinMPQ.

Finally, to be able to view models, skins and your changes, download the Warcraft 3 Viewer.